Oxide, sulfide, selenide or telluride of rare-earth, which is a chalcogen semi conductor, is an important fundamental material used in various fields such as luminescent material, optoelectronics material, photo-magnetic material or magnetic material. Particularly, EuO or EuS which uses low valence Eu (II) (di-valent) are important material as a photo-magnetic material which displays magnetic-optical Faraday effect and Kerr effect.
In the meanwhile, di-valent Eu (II) ion can not exist stable and easily oxidized to tri-valent Eu (III). Therefore, in the ordinary condition, it is not possible to use Eu (II) salt as a material to synthesis EuO. So, conventionally, a thermo structural rearrangement method characterizing to treat Eu metal or Eu (III) salt by high temperature higher than 1000° C. is used for the synthesis of low valence EuO. [edited by Ginya Adachi “Chemistry of rare-earth” Kagaku Dojin, p398 (1999)].
Further, the method to prepare said compound by reacting Eu metal with liquid ammonia (−77° C.) is proposed [D. E. Eastman, F. Holtzberg and S. Methferrel, Phey. Rev. Lett., 23, 5, 226 (1969)]. However, above mentioned methods are belonging to high energy process, further, according to said preparation methods, it is difficult to adjust the particle size of generated Eu (II) semi-conductor. Therefore, the diameter of the finest one among the obtained Eu (II) semi-conductor is several μm, and it is necessary to make the atmosphere temperature to very low temperature (6K) to generate photo-magnetic property.
In general, when the size of a solid particle is minimized to the order of nano meter, the quantum size effect appears. Therefore, the Eu (II) semi-conductor whose particle size is minimized to nano meter is predicted to generate specific property such as hard magnetism, strong luminescent or photo-magneticity at the room temperature. However, up to the present, it is recognized that the preparation of Eu (II) semi-conductor of low valence having above mentioned specific properties is impossible.
While, there is a paper reporting that when an UV-ray (254 nm) is irradiated to Eu (III) ion in methanol, Eu (II) ion can be generated [document: M.Kusaba, N.Nakashima, W. Awamura, Ylzawa and C. Yamanaka, Chem. Phys. Lett., 197.136(1992)].
However, there are no paper reporting the preparation of oxide or sulfide compound of Eu (II) of low valence and nano size, further, there are also no paper reporting the preparation of Eu (II) semi-conductor by ray irradiation utilizing above mentioned photo-chemical reaction.
The object to be dissolved by the present invention is to provide a crystalline particle of (chalcogenide compound of low valence) oxide or sulfide of Eu (II) which generates quantum efficiency, especially to provide stable crystalline particles of nano size having narrow particle size distribution, further to provide a method for producing said crystalline particles.
The inventors of the present invention have carried out earnest study to solve above mentioned object. And, have investigated to apply the phenomenon disclosed in above mentioned document to the production of oxide or sulfide of nano size Eu (II), namely, said phenomenon is that: when an VV-ray shorter than 300 nm, for example, an UV-ray by a fluorescent lamp which radiates ray of 254 nm is irradiated to Eu (III) ion in methanol, Eu (II) ion can be generated. During the study, salts which supply Eu (III) ion, reagents forming the reaction atmosphere such as solvent which generate Eu (II) ion, generated EU (II) ion are investigated, particularly, the preferable combinations of oxygen or sulfur atoms supplying compounds which provide the environment to form oxide or sulfide are investigated.
In above mentioned investigation, various agents mentioned below were used.    1. As the salt to supply said Eu (III) ion, salt of inorganic acid, especially nitrate (6 hydrate) and salt of organic acid such as salts of mono-carboxylic acid, di-carboxylic acid or tri-carboxylic acid are used.    2. As the solvent to form the reaction atmosphere, polar solvent which dissolve above mentioned salts, for example, alcoholic solvent such as methanol or ethanol or ether are used.    3. At the irradiation of light to said reaction system, a compound to generate a radical which makes Eu (III) reduce to Eu (II), such as methanol (in this case methanol also acts as a solvent) or water (e.g. crystal water of inorganic salt) are coexisted and used as an agent. Still more,    4. The oxygen or sulfur supplying compound generated by said reduction which reacts rapidly with Eu (II), for example, urea as the supplying compound of oxygen or thio urea as the supplying compound for sulfur are used as the coexisted agents.
Under said atmospheric condition for reaction, when the light from the ultra violet region to the visible region, for example the light of 200 nm to 1000 m wavelength region is irradiated, EuO or EuS are generated as the very fine particle whose particle size is equal to or smaller than 10 nm, equal to or larger than 4 nm, and having narrow particle size distribution. That is, the object of the present invention is dissolved. As a light source, laser beam can be used, further the light source of longer wavelength region which brings multiphoton excitation can be used as a light source. Furthermore, the compound which indicates photosensitizing effect can be used together with.